Lithium secondary battery including nonaqueous electrolyte having lithium-ion conductivity

ABSTRACT

A lithium secondary battery comprises an electrode group and a nonaqueous electrolyte having lithium-ion conductivity. A negative electrode current collector has a first surface facing outward of winding of the electrode group and a second surface facing inward of the winding of the electrode group. At least the first surface or the second surface includes a first region and a second region that is closer to an innermost circumference of the winding of the electrode group than the first region. Protrusions include outer-circumference-side protrusions disposed on the first region and inner-circumference-side protrusions disposed on the second region. In at least the first surface or the second surface, a first area rate is larger than a second area rate.

BACKGROUND 1. Technical Field

The present disclosure relates to a lithium secondary battery includinga nonaqueous electrolyte having lithium ion-conductivity.

2. Description of the Related Art

Nonaqueous electrolyte secondary batteries are used in, for example,information and communication technologies (ICT), such as personalcomputers and smart phones, cars, and power storage. In these uses, thenonaqueous electrolyte secondary batteries are required to have furtherhigher capacities. As high-capacity nonaqueous electrolyte secondarybatteries, lithium-ion batteries are known. The capacity of alithium-ion battery can be increased by using, for example, acombination of graphite and an alloy active material, such as a siliconcompound, as the negative electrode active material. However, theincrease in the capacity of a lithium-ion battery is reaching a limit.

Lithium secondary batteries are promising as nonaqueous electrolytesecondary batteries having a capacity higher than that of lithium-ionbatteries. In lithium secondary batteries, lithium metal is deposited onthe negative electrode during charging, and this lithium metal isdissolved in a nonaqueous electrolyte during discharging.

In lithium secondary batteries, it has been investigated to improve, forexample, the shape of the negative electrode current collector from theviewpoint of reducing deterioration of the battery characteristics dueto deposition of lithium metal in a dendrite form. For example, JapaneseUnexamined Patent Application Publication No. 2001-243957 (PTL 1)proposes to control the ten-point average roughness Rz of the lithiummetal deposition surface of the negative electrode current collector to10 μm or less. Japanese Unexamined Patent Application Publication(Translation of PCT Application) No. 2016-527680 (PTL 2) proposes alithium secondary battery using a negative electrode including a porousmetal current collector and lithium metal inserted into the pores of thecurrent collector. Japanese Unexamined Patent Application PublicationNo. 2006-156351 (PTL 3) proposes a lithium metal polymer secondarybattery using a negative electrode current collector having a surfaceprovided with a plurality of recesses having a prescribed shape.

SUMMARY

One non-limiting and exemplary embodiment provides a lithium secondarybattery including a wound electrode group that reduces expansion of thenegative electrode during charging.

In one general aspect, the techniques disclosed here feature a lithiumsecondary battery comprising an electrode group and a nonaqueouselectrolyte having lithium-ion conductivity. The electrode groupincludes a positive electrode, a negative electrode, and a separatordisposed between the positive electrode and the negative electrode. Thepositive electrode contains a positive electrode active materialcontaining lithium. The negative electrode includes a negative electrodecurrent collector and protrusions disposed on the negative electrodecurrent collector. The positive electrode, the negative electrode, andthe separator of the electrode group are wound. Lithium metal isdeposited on the negative electrode during charging, and the lithiummetal is dissolved in the nonaqueous electrolyte during discharging. Thenegative electrode current collector has a first surface facing outwardof the winding of the electrode group and a second surface facing inwardof the winding of the electrode group. At least the first surface or thesecond surface includes a first region and a second region that iscloser to an innermost circumference of the winding of the electrodegroup than the first region. The protrusions includeouter-circumference-side protrusions disposed on the first region andinner-circumference-side protrusions disposed on the second region. Inat least the first surface or the second surface, a first area rate islarger than a second area rate. The first area rate refers to(A_(OP)/A_(O))×100%. A_(OP) refers to a sum of the projection areas ofthe outer-circumference-side protrusions onto the first region. A_(O)refers to an area of the first region. The second area rate refers to(A_(IP)/A_(I))×100%. A_(IP) refers to a sum of the projection areas ofthe inner-circumference-side protrusions onto the second region. A_(I)refers to an area of the second region.

According to embodiments of the present disclosure, in a lithiumsecondary battery including a wound electrode group, expansion of thenegative electrode caused by charging can be reduced.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view schematically illustrating a negative electrodeused in a lithium secondary battery according to an embodiment of thepresent disclosure;

FIG. 2A is a cross-sectional view of the section taken along the lineIIA-IIA in FIG. 1 viewed from the direction of the arrow;

FIG. 2B is a cross-sectional view of the section taken along the lineIIB-IIB in FIG. 1 viewed from the direction of the arrow;

FIG. 3 is a plan view schematically illustrating another negativeelectrode used in a lithium secondary battery according to an embodimentof the present disclosure;

FIG. 4A is a cross-sectional view of the section taken along the lineIVA-IVA in FIG. 3 viewed from the direction of the arrow;

FIG. 4B is a cross-sectional view of the section taken along the lineIVB-IVB in FIG. 3 viewed from the direction of the arrow;

FIG. 5 is a longitudinal cross-sectional view schematically illustratinga lithium secondary battery according to another embodiment of thepresent disclosure;

FIG. 6 is an enlarged cross-sectional view schematically illustratingthe region indicated by VI in FIG. 5; and

FIG. 7 is an enlarged cross-sectional view schematically illustratingthe region indicated by VII in FIG. 5.

DETAILED DESCRIPTION

Underlying Knowledge Forming Basis of the Present Disclosure

An embodiment of the present disclosure relates to a lithium secondarybattery using lithium metal as a negative electrode active material andincluding a wound electrode group. More specifically, the embodiment ofthe present disclosure relates to an improvement of the negativeelectrode current collector in the wound electrode group. Lithiumsecondary batteries are also called lithium-metal secondary batteries.In lithium secondary batteries, lithium metal may be deposited in adendrite form at the negative electrode during charging. Furthermore,generation of dendrites increases the specific surface area of thenegative electrode, which may increase a side reaction. Accordingly, thedischarge capacity and cycle characteristics tend to decrease. Regardingthis, PTL 1 teaches that the generation of dendrites is reduced bycontrolling the ten-point average roughness Rz of the lithium metaldeposition surface of the negative electrode to 10 μm or less to give ahigh charge-discharge efficiency.

In addition, in lithium secondary batteries, since lithium metal isdeposited on the negative electrode during charging, the amount ofexpansion of the negative electrode is particularly apt to increase. Inthe present specification, the term “expansion of negative electrode”means that the sum of the volume of the negative electrode and thevolume of deposited lithium metal is increased. In particular, whenlithium metal is deposited in a dendrite form, the amount of expansionis further increased. In a cylindrical lithium battery including a woundelectrode group, a stress is generated by excessive expansion of thenegative electrode. In order to absorb the change in the volume of thenegative electrode by charge and discharge, PTL 2 proposes use of, forexample, a porous copper or nickel negative electrode current collectorhaving a pore rate of 50% to 99% and a pore size of 5 to 500 μm. In thenegative electrode current collector of PTL 3, recesses are provided forsecuring spaces for forming lithium metal in a dendrite form.

The stress caused by deposition of lithium metal is released from, forexample, the main surface and the side surface of the negative electrodein a coin-shaped electrode group, and the stress is released from, forexample, the end of the negative electrode in a laminated electrodegroup. In contrast, in a wound electrode group, the stress caused bydeposition of lithium metal is a tensile strain in the circumferentialdirection in a cross section perpendicular to the winding axis of theelectrode group. In the wound electrode group, the stress caused bydeposition of lithium metal is less likely released from the innercircumference side of the electrode group and the end of the negativeelectrode, and therefore the stress heads for the outer circumferenceside of the electrode group. In addition, when the winding end of thewound electrode group is fixed with tape and is surrounded by a batterycase, a stress is also applied from the outside. Thus, in a woundelectrode group, the stress is less likely dispersed, compared to otherelectrode groups such as coin-shaped or laminated electrode groups, andexcessive expansion or uneven expansion of the negative electrode tendsto occur.

In the present specification, the negative electrode current collectorof the wound electrode group includes a first surface facing outward ofthe winding of the electrode group and a second surface facing inward ofthe winding of the electrode group. That is, the first surface faces thedirection going away from the winding axis of the electrode group withrespect to the negative electrode current collector, and the secondsurface faces the direction approaching the winding axis of theelectrode group with respect to the negative electrode currentcollector. Hereinafter, in a negative electrode current collector, theside facing outward of the winding of the electrode group may bereferred to as the outside, and the side facing inward of the winding ofthe electrode group may be referred to as the inside. If at least theouter surface or the inner surface of the negative electrode currentcollector includes a first region and a second region that is closer tothe innermost circumference of the winding of the electrode group thanthe first region, in the electrode group, the portion including thefirst region is referred to as the outer winding portion, and theportion including the second region is referred to as the inner windingportion.

The pressure applied to the surface of the negative electrode currentcollector by the stress heading for the outer circumference side asdescribed above and the stress from the outside of the electrode groupis higher at the outer winding portion than that at the inner windingportion of the electrode group. Hereinafter, the pressure applied to thesurface of the negative electrode current collector may be referred toas surface pressure. In the wound electrode group, as described above,large stress and surface pressure are applied to the outer windingportion of the electrode group. Accordingly, the pressure applied to thelithium metal deposited on the negative electrode surface at the outerwinding portion is larger than that applied to the lithium metaldeposited on the negative electrode surface of the inner windingportion. Due to this large pressure, the lithium metal deposited on thenegative electrode surface is compressed in the outer winding portion ofthe electrode group. In contrast, in the inner winding portion of theelectrode group, the lithium metal deposited on the negative electrodesurface is less likely compressed, and the thickness of lithium metal islarger than that in the outer winding portion.

Due to such differences in the stress and surface pressure between theinner winding portion and the outer winding portion of the electrodegroup, deposition of lithium metal on the negative electrode surfacetends to be uneven, and therefore the negative electrode may locallyexpand excessively. In addition, the charge-discharge efficiency maydecrease.

In the negative electrode current collector of PTL 2 or PTL 3, lithiummetal is deposited in the spaces of pores or recesses by charging. InPTL 2 and PTL 3, basically, laminated or coin-shaped electrode groupsare supposed. Accordingly, the pressure generated in the electrode groupis less likely applied to the lithium metal in the pores or therecesses. Even if the negative electrode current collector of PTL 2 orPTL 3 is used in a wound electrode group, uneven deformation tends tooccur by winding. As a result, the stress applied to the depositedlithium metal is uneven, and therefore the expansion of the negativeelectrode during charging tends to be uneven. Accordingly, it isdifficult to sufficiently reduce the expansion of the negative electrodeduring charging. In addition, the stress is less likely applied to thelithium metal in the pores or the recesses, and therefore the lithiummetal tends to peel from the wall surface of the current collector. Thepeeled lithium metal cannot be dissolved during discharging, andtherefore the charge-discharge efficiency is decreased.

The present inventors diligently studied to solve the above-mentionedproblems and, as a result, arrived at the lithium secondary batteryaccording to the present disclosure. The lithium secondary batteryaccording to one aspect of the present disclosure comprises an electrodegroup and a lithium ion conductive nonaqueous electrolyte. The electrodegroup includes a positive electrode, a negative electrode, and aseparator disposed between the positive electrode and the negativeelectrode. The positive electrode includes a positive electrode activematerial containing lithium. The negative electrode includes a negativeelectrode current collector and protrusions disposed on the negativeelectrode current collector. The positive electrode, the negativeelectrode, and the separator are wound. Lithium metal is deposited onthe negative electrode during charging, and the lithium metal isdissolved in the nonaqueous electrolyte during discharging. Theprotrusions include outer-circumference-side protrusions present in theouter winding portion of the electrode group andinner-circumference-side protrusions present in the inner windingportion of the electrode group. The first area rate of theouter-circumference-side protrusions to the outer winding portion islarger than the second area rate of the inner-circumference-sideprotrusions to the inner winding portion.

In the present disclosure, the “first area rate of theouter-circumference-side protrusions to the outer winding portion” meansthe rate (A_(OP)/A_(O))×100% of the sum (A_(OP)) of projection areas ofthe outer-circumference-side protrusions onto the first region to thearea (A_(O)) of the first region in the first or second surface of thenegative electrode current collector. The “second area rate of theinner-circumference-side protrusions to the inner winding portion” meansthe rate (A_(IP)/A_(I))×100% of the sum (A_(IP)) of the projection areasof the inner-circumference-side protrusions onto the second region to(A_(I)) the area of the second region in the first or second surface ofthe negative electrode current collector.

According to the above-described aspect of the present disclosure, inthe wound electrode group, a negative electrode including a negativeelectrode current collector and protrusions disposed on the negativeelectrode current collector is used. The protrusions can secure spacesfor depositing lithium metal at the negative electrode, and thereforethe change in the apparent volume of the negative electrode caused bydeposition of lithium metal can be decreased. In the presentspecification, the term “apparent volume of a negative electrode” meansthe total volume of the negative electrode, the deposited lithium metal,and the spaces secured by the protrusions.

Furthermore, the first area rate of the outer-circumference-sideprotrusions to the outer winding portion is controlled to be larger thanthe second area rate of the inner-circumference-side protrusions to theinner winding portion. In other words, by controlling the second arearate of the inner-circumference-side protrusions to the inner windingportion to be less than the first area rate of theouter-circumference-side protrusions to the outer winding portion, evenif the thickness of the lithium metal deposited by charging is increasedin the inner winding portion, this increase in the volume can beeffectively absorbed by the space between the inner-circumference-sideprotrusions. Accordingly, the increase in the apparent volume of thenegative electrode can be further reduced. Thus, in each of the outerwinding portion and the inner winding portion of the electrode group, aspace having a volume suitable for the thickness of lithium metal to bedeposited by charging can be previously secured by the protrusions.Accordingly, it is unnecessary to decrease the volume of the negativeelectrode and/or the positive electrode at an early stage inanticipation of expansion of the negative electrode. As a result, a highdischarge capacity can be readily secured. In addition, even if lithiummetal is generated in a dendrite form, the lithium metal can beaccommodated in the space formed in the negative electrode by theprotrusions.

Since the electrode group is a winding type, a certain degree ofpressure is applied to the lithium metal deposited in the space in thenegative electrode. Accordingly, the lithium metal deposited in thespace is less likely peeled off, unlike the cases of PTL 2 and PTL 3.Consequently, the deterioration of the charge-discharge efficiency canalso be reduced. In addition, since an appropriate pressure is appliedto the deposited lithium metal, the deposition itself of lithium metalin a dendrite form can be reduced even if the negative electrode is notsmoothened, unlike the case of PTL 1.

For example, in each of the outer surface and the inner surface (i.e.,the first surface and the second surface) of the negative electrodecurrent collector, the region facing the positive electrode activematerial is divided into two portions by the center line in thelongitudinal direction of the region. Furthermore, for example, theportion far from the innermost circumference of the winding of theelectrode group than the center line is defined as a first regionlocated in the outer winding portion of the electrode group, and theportion closer to the innermost circumference of the winding of theelectrode group than the center line is defined as a second regionlocated in the inner winding portion of the electrode group.

The negative electrode current collector usually includes a firstsurface and a second surface which is on the opposite side of the firstsurface. The first surface and the second surface mean two main surfacesof, for example, a sheet-like negative electrode current collector. Theouter-circumference-side protrusions and the inner-circumference-sideprotrusions are disposed on at least the first surface side or thesecond surface side. The first surface may be the outer surface of thenegative electrode current collector. The second surface may be theinner surface of the negative electrode current collector. From theviewpoint of capable of securing spaces for deposition of lithium metalduring charging on the first surface and the second surface of thenegative electrode current collector and in the vicinities thereof, theouter-circumference-side protrusions may include first protrusionsdisposed on the first surface side and second protrusions disposed onthe second surface side, and the inner-circumference-side protrusionsmay include first protrusions disposed on the first surface side andsecond protrusions disposed on the second surface side. The firstprotrusions protrude from the first surface side of the negativeelectrode current collector toward the surface of the separator facingthe first surface. The second protrusions protrude from the secondsurface side of the negative electrode current collector toward thesurface of the separator facing the second surface.

In this case, the outer-circumference-side protrusions are protrusionsdisposed on the outer circumference side first surface and the outercircumference side second surface located in the outer winding portion(i.e., the first region of the first surface and the first region of thesecond surface). The first area rate may be the average of the area rate(an example of a third area rate (A_(O1P)/A_(O1))×100%) of the totalprojection area (an example of a sum A_(O1P)) of the protrusionsdisposed on the outer circumference side first surface onto the outercircumference side first surface to the area (an example of an areaA_(O1)) of the outer circumference side first surface and the area rate(an example of a fourth area rate (A_(O2P)/A_(O2))×100%) of the totalprojection area (an example of a sum A_(O2P)) of the protrusionsdisposed on the outer circumference side second surface onto the outercircumference side second surface to the area (an example of an areaA_(O2)) of the outer circumference side second surface. Theinner-circumference-side protrusions are protrusions disposed on theinner circumference side first surface and the inner circumference sidesecond surface located in the inner winding portion (i.e., the secondregion of the first surface and the second region of the secondsurface). The second area rate may be the average of the area rate (anexample of a fifth area rate (A_(I1P)/A_(I1))×100%) of the totalprojection area (an example of a sum A_(I1P)) of the protrusionsdisposed on the inner circumference side first surface onto the innercircumference side first surface to the area (an example of an areaA_(I1)) of the inner circumference side first surface and the area rate(an example of a sixth area rate (A_(I2P)/A_(I2))×100%) of the totalprojection area (an example of a sum A_(I2P)) of the protrusionsdisposed on the inner circumference side second surface onto the innercircumference side second surface to the area (an example of an areaA_(I2)) of the inner circumference side second surface.

Hereinafter, the total area of the projection shapes (i.e., projectionareas) obtained by projecting the protrusions onto the surface of thenegative electrode current collector on which the protrusions aredisposed may be referred to as the area of the protrusions. Theprojection shapes of protrusions onto the surface of the negativeelectrode current collector are the shapes formed when the protrusionsare projected onto the surface of the negative electrode currentcollector on the side on which the protrusions are formed in thethickness direction of the negative electrode current collector.Although some layer may be formed between the negative electrode currentcollector and the protrusions, the projection shapes or the projectionareas may be determined by assuming that the protrusions are projectedonto the surface of the negative electrode current collector. The areaof each of the first and second regions and the areas of protrusions maybe determined from the negative electrode in a state where the firstsurface and the second surface are planarly spread. The negativeelectrode before being formed into a wound electrode group may be usedfor determining each area. When the protrusions are linear as describedbelow and are aligned in substantially parallel to each other, the rateof the area of the protrusions can also be estimated from the clearancebetween two adjacent protrusions and the width of the protrusions.

However, in the calculation of the first area rate and the second arearate, it is not necessary to consider the region of the surface of thenegative electrode current collector not facing the positive electrodeactive material. That is, the first and second regions do not includethe region of the surface of the negative electrode current collectornot facing the positive electrode active material. Accordingly, the areaof the first and second regions does not include the area of the regionof the surface of the negative electrode current collector not facingthe positive electrode active material. In the wound electrode group,for example, there are cases in which in the outermost circumference ofthe winding, a region on the outside of the negative electrode currentcollector does not face the positive electrode active material. In sucha case, since lithium metal is less likely deposited on the region onthe outside not facing the positive electrode active material, theregion is not considered in the calculation of the first area rate.Similarly, there are cases in which in the innermost circumference ofthe winding, a region on the inside of the negative electrode currentcollector does not face the positive electrode active material. In thiscase, since lithium metal is less likely deposited on the region on theinside not facing the positive electrode active material, the region isnot considered in the calculation of the second area rate.

When the width of the negative electrode current collector in thedirection parallel to the winding axis is larger than the width of thepositive electrode current collector, at the upper end and/or the lowerend (i.e., one end portion and/or the other end portion in the directionparallel to the winding axis) of the electrode group, a band-shapedregion of the negative electrode current collector extending in thelongitudinal direction perpendicular to the winding axis does not facethe positive electrode active material. In this case, the band-shapedregion is not considered in the calculation of each area rate. Thelongitudinal direction of the region of the negative electrode currentcollector facing the positive electrode active material is parallel tothe longitudinal direction of the negative electrode current collector.In each of the first surface and the second surface of the negativeelectrode current collector, the direction perpendicular to the windingaxis of the electrode group is defined as the longitudinal direction ofthe negative electrode current collector, and the direction parallel tothe winding axis is defined as the width direction of the negativeelectrode current collector. Hereinafter, the longitudinal direction ofthe negative electrode current collector is referred to as the firstlongitudinal direction, and the width direction is referred to as thefirst width direction. More specifically, on both ends of the negativeelectrode current collector in the longitudinal direction, the lineconnecting the midpoints in the width direction is defined as the firstcenter line, and the direction of the first center line is defined asthe first longitudinal direction. The direction perpendicular to thefirst longitudinal direction is defined as the first width direction.

The difference between the first area rate and the second area rate canbe adjusted according to, for example, the energy density and the sizeof the battery, as long as the first area rate is larger than the secondarea rate. The difference between the first area rate and the secondarea rate may be 3 percentage points or more and 5 percentage points ormore. When the difference is within such a range, even if the thicknessof the lithium metal deposited on the inner circumference side of theelectrode group is large, the change in the volume of the negativeelectrode by this deposition can be readily absorbed. The differencebetween the first area rate and the second area rate is, for example, 50percentage points or less and may be 20 percentage points or less. Whenthe difference is within such a range, since a space having a volumesuitable for the deposition amount of lithium tends to be secured, ahigher discharge capacity tends to be secured while maintaining theeffect of reducing expansion of the negative electrode. These lowerlimit value and upper limit value can be arbitrarily combined. Thedifference between the first area rate and the second area rate is thevalue obtained by subtracting the second area rate from the first arearate.

The first area rate and the second area rate may each be 0.2% or more,1% or more, or 3% or more. When the rates are within such ranges, theseparator can be readily supported by the protrusions, and the distancebetween the negative electrode current collector and the separator canbe readily made constant. Accordingly, the effect of reducing expansionof the negative electrode can be further enhanced. It is also possibleto enhance the effect of homogeneously performing the charge-dischargereaction. The first area rate and the second area rate may each be 70%or less or 50% or less. When the rates are within such ranges, since aspace tends to be secured between the surface of the negative electrodecurrent collector and the separator, expansion of the negative electrodecaused by deposition of lithium metal can be reduced while securing ahigh capacity. These lower limit value and upper limit value can bearbitrarily combined.

A structure of the lithium secondary battery according to theabove-described aspect will now be more specifically described withreference to the drawings as appropriate. First of all, the structure ofthe negative electrode will be described.

Negative Electrode

The negative electrode includes a negative electrode current collectorand protrusions disposed on the negative electrode current collector.The negative electrode current collector usually includes a firstsurface and a second surface on the opposite side of the first surface.The first surface and the second surface are the outer surface and theinner surface, respectively, of the negative electrode current collectorin a wound electrode group. In the negative electrode of a lithiumsecondary battery, lithium metal is deposited by charging. Morespecifically, lithium ions contained in a nonaqueous electrolyte receiveelectrons on the negative electrode by charging and are converted intolithium metal to be deposited on the surface of the negative electrode.The deposited lithium metal is dissolved as lithium ions in thenonaqueous electrolyte by discharging. The lithium ions contained in thenonaqueous electrolyte may be derived from a lithium salt added to thenonaqueous electrolyte, may be supplied from the positive electrodeactive material by charging, or may be both.

The negative electrode can secure a space for accommodating the lithiummetal deposited on the surface of the negative electrode by havingprotrusions. Accordingly, the space can reduce expansion of the negativeelectrode caused by deposition of lithium metal. In addition, the secondarea rate of the inner-circumference-side protrusions to the innerwinding portion is controlled to be smaller than the first area rate ofthe outer-circumference-side protrusions to the outer winding portion.Consequently, as described above, even if the thickness of lithium metaldeposited on the inner winding portion by charging is large, this volumeincrease can be effectively absorbed. Accordingly, the increase in theapparent volume of the negative electrode can be further reduced.

The heights of the outer-circumference-side protrusions and theinner-circumference-side protrusions (hereinafter, may be simplyreferred to as protrusions) can be respectively determined according tothe positions where the protrusions are formed and the deposition amountof lithium metal. The average height of the outer-circumference-sideprotrusions may be 15 μm or more, 20 μm or more, or 30 μm or more.Furthermore, the average height of the outer-circumference-sideprotrusions may be 40 μm or more or 50 μm or more. The average height ofthe inner-circumference-side protrusions may be 15 μm or more, 20 μm ormore, or 30 μm or more. Furthermore, the average height of theinner-circumference-side protrusions may be 40 μm or more or 50 μm ormore. When the average height is within such a range, the effect ofabsorbing a large stress applied to the outer winding portion and theeffect of absorbing the change in the volume of the negative electrodecaused by deposition of lithium metal can be further enhanced. Theeffect of reducting damage of the electrode can also be enhanced.

The average height of the outer-circumference-side protrusions may be120 μm or less or 110 μm or less. Furthermore, the average height of theouter-circumference-side protrusions may be 100 μm or less or 90 μm orless. The average height of the inner-circumference-side protrusions maybe 120 μm or less or 110 μm or less. Furthermore, the average height ofthe inner-circumference-side protrusions may be 100 μm or less or 90 μmor less. When the average height is within such a range, the separatorappropriately presses the lithium metal deposited on the negativeelectrode surface to increase the conductivity between the lithium metaland the negative electrode current collector, resulting in enhancementin the charge-discharge efficiency. In addition, excessive pressing onthe protrusions by the separator is reduced to protect the electrode.These lower limit value and upper limit value can be arbitrarilycombined.

In terms of ease of manufacturing, the difference between the firstaverage height of the outer-circumference-side protrusions and thesecond average height of the inner-circumference-side protrusions may beless than 3% of the second average height. That is, the average heightsof the outer-circumference-side protrusions and theinner-circumference-side protrusions may be substantially the same. Froma similar viewpoint, the difference between the average height of thefirst protrusions and the average height of the second protrusions maybe less than 3% of the average height of the second protrusions. Thatis, the average heights of the first protrusions and the secondprotrusions may be substantially the same.

The average height of protrusions can be determined by, for example,arbitrarily selecting three protrusions in a cross-section photograph ofthe negative electrode in the thickness direction, measuring thedistance from the end of each of the selected protrusions on thenegative electrode current collector side to the end of the protrusionon the side opposite to the negative electrode current collector as theheight of each protrusion, and averaging the heights of theseprotrusions. Alternatively, the first average height may be determinedby cutting out a certain area (e.g., 5 cm²) or a plurality of arbitraryregions in the first region of the negative electrode current collectorand averaging the heights of a plurality of arbitrary protrusionspresent in the certain area or the arbitrary regions. In such a case,the first average height may be determined by taking a plurality ofcross-section photographs in a certain area or a plurality of arbitraryregions, measuring the height of each protrusion in these cross-sectionphotographs, and averaging the heights of the protrusions. Theprotrusions as the measurement object may be disposed over the entiresurface of the first region or may be disposed only in a very smallportion.

In measurement of the average height of protrusions, when the end ofeach of the protrusions on the negative electrode current collector sideand/or the end on the opposite side is not flat, the maximum value ofthe length between both ends of each protrusion in the directionparallel to the thickness direction of the negative electrode is definedas the height of each protrusion. In addition, in measurement of theaverage height of protrusions, when protrusions are formed on both thefirst surface and the second surface of the negative electrode currentcollector, three protrusions are arbitrarily selected from theprotrusions formed on each of the first surface and the second surface.Each of the average heights may be determined based on a cross-sectionphotograph of the electrode group that allows observation of thecross-section in the thickness direction of the negative electrode.

When the first surface and/or the second surface is rough, the surfaceroughness Rz of the first surface and/or the second surface may be 1 μmor less. Each height of the protrusions on the first surface and/or thesecond surface may be higher than 1 μm. When the first surface and/orthe second surface is rough and the protrusions and the negativeelectrode current collector are integrally made of the same material,each height of the protrusions on the first surface and/or the secondsurface may be measured based on the bottom of the rough. In this case,the height is measured in a state in which the first surface and thesecond surface are extended to be flat by unwinding the electrode group.

A least a part of the protrusions may be in contact with the separator.For example, the first protrusions may be in contact with the surface ofthe separator facing the first surface. The second protrusions may be incontact with the surface of the separator facing the second surface. Thepresence of the protrusions secures a space between the negativeelectrode and the separator. In such a case, lithium metal is depositedby charging in the space formed between the negative electrode currentcollector and the separator. The influence of the relationship betweenthe first area rate and the second area rate is remarkably exhibited bythe contact between the protrusions and the separator, and the effect ofreducing expansion of the negative electrode can be enhanced. Inaddition, since the deposition of lithium metal is reduced at theportion where each protrusion is in contact with the separator, such asthe tip of the protrusion, it is also possible to reduce local expansionof the negative electrode.

From the viewpoint of further enhancing the effect of reducing expansionof the negative electrode, in each of the first surface and the secondsurface of the negative electrode current collector, 80% or more of thetotal projection area of the protrusions onto the surface of thenegative electrode current collector may be in contact with theseparator. From the same viewpoint, all of the protrusions formed oneach of the first surface and the second surface may be in contact withthe separator. The projection shape of each of the protrusions onto thesurface of the negative electrode current collector is not particularlylimited. From the viewpoint of easily supporting the separator andeasily supplying a nonaqueous electrolyte to the vicinity of anelectrode, the projection shape of each of the protrusions onto thesurface of the negative electrode current collector may be a line shape.The term “line shape” includes a strip shape. The strip shape refers toa line shape having a relatively short length.

From the viewpoint of securing a space having an appropriate volume foraccommodating deposited lithium metal, in the direction parallel to thesurface of the negative electrode, two adjacent protrusions out of theprotrusions may be spaced to some extent. For example, the minimumclearance between two adjacent protrusions may be larger than themaximum width of the two adjacent protrusions. The minimum clearancebetween two adjacent protrusions means the smallest distance betweenouter edges of the projection shapes of two adjacent protrusionsarbitrarily selected from protrusions when the two adjacent protrusionsare projected onto the surface of the negative electrode currentcollector on the side on which the protrusions are formed in thethickness direction of the negative electrode current collector. Themaximum width of two adjacent protrusions is the maximum value of thewidth (the length in the direction perpendicular to the longitudinaldirection of the protrusions) of the projection shapes of the twoadjacent protrusions onto the surface of the negative electrode currentcollector on the side on which the protrusions are formed. When theprojections have circular shapes, the larger one of the diameters of theprojection shapes of two protrusions is defined as the maximum width.

In each of the first surface and the second surface of the negativeelectrode current collector, the projection shapes of the protrusionsonto the surface of the negative electrode current collector may be eacha line shape, and the protrusions may be aligned approximately parallelto each other in the longitudinal direction. In such a case, the minimumclearance between two adjacent protrusions may be larger than themaximum width of the two adjacent protrusions. In such a case, theseparator is readily supported by the protrusions, and a space having anappropriate volume tends to be secured between two adjacent protrusions.Hereinafter, the longitudinal direction of each protrusion of which theprojection shape is line like is referred to as the second longitudinaldirection. More specifically, on two ends in the longitudinal directionof a protrusion of which the projection shape is line like, the centerline connecting the midpoints in the respective width directions isdefined as the second center line, and the direction of the secondcenter line is defined as the second longitudinal direction.

The state in which the protrusions are aligned approximately parallel toeach other in the second longitudinal direction refers to a case inwhich the protrusions are parallel to each other in the secondlongitudinal direction or the acute angle formed by the secondlongitudinal directions of the protrusions is 30° or less. Thelongitudinal direction of the projection shape formed by projecting eachprotrusion onto the surface of the negative electrode current collectoron which the protrusion is formed in the thickness direction of thenegative electrode current collector is defined as the secondlongitudinal direction of each protrusion.

The minimum clearance is not limited as long as the value is larger thanthe minimum width of the protrusions and may be 150% or more, 400% ormore, or 500% or more the minimum width. The minimum clearance may be3000% or less of the minimum width of the protrusions.

When line-shaped protrusions are aligned approximately parallel to eachother, the clearance between the protrusions may be determined from thecenter-to-center distance of two adjacent protrusions and the width ofeach protrusion. The center of a protrusion in this case refers to thesecond center line of the protrusion. The distance between the secondcenter lines of two adjacent protrusions may be defined as thecenter-to-center distance.

In order to make the nonaqueous electrolyte easily penetrate into theinside of the electrode group, in the first surface and/or the secondsurface, no continuous frame-like protrusion surrounding the entire or apart of each surface may be formed. In the periphery of the firstsurface and/or the second surface, no continuous frame-like protrusionsurrounding most of each surface may be formed. When a continuousframe-like protrusion is not formed, the nonaqueous electrolyte tends topenetrate into the inside from the portion where no protrusions areformed, and the separator tends to become into contact with thedeposited lithium metal. Accordingly, the effect of reducing unevendeposition of lithium metal is enhanced, and therefore generation ofdendrites can be reduced and deterioration in the charge-dischargeefficiency can be reduced.

The first surface and/or the second surface may include a band-shapedregion where no protrusions are formed along at least the firstlongitudinal direction or the first width direction. Each surface mayinclude at least one band-shaped region or may include two or moreband-shaped regions. In such a case, the nonaqueous electrolyte tends topenetrate into the inside of the electrode group through the band-shapedregion. Since the nonaqueous electrolyte can be easily held between thepositive electrode and the negative electrode, deposition anddissolution of lithium metal smoothly progress to reduce deteriorationof the capacity and the charge-discharge efficiency. In addition, in theband-shaped region, the separator tends to come into contact with thedeposited lithium metal. Consequently, since the effect of reducinguneven deposition of lithium metal is enhanced, generation of dendritescan be reduced.

The band-shaped region may be formed along the first longitudinaldirection or the first width direction. In the first surface and/or thesecond surface, the negative electrode current collector may have aband-shaped region (first band-shaped region) along one of the firstlongitudinal direction and the first width direction and a band-shapedregion (second band-shaped region) along the other direction. From theviewpoint of allowing the nonaqueous electrolyte to easily penetratefurther into the inner circumference side of the wound electrode groupand easily securing a high capacity and a high charge-dischargeefficiency, a first band-shaped region may be provided along the firstlongitudinal direction. The first band-shaped region is easily formed byproviding protrusions whose projection shapes onto each surface of thenegative electrode current collector are line shapes on each surface. Inparticular, the first band-shaped region is easily formed between twoadjacent protrusions in the first width direction by providingprotrusions such that the protrusions are approximately parallel to thefirst longitudinal direction in the second longitudinal direction.

The expression “a first band-shaped region is provided along the firstlongitudinal direction” means that a band-shaped region where noprotrusions are formed is present on the negative electrode currentcollector in a direction approximately parallel to the firstlongitudinal direction. The expression “a second band-shaped region isprovided along the first width direction” means that a band-shapedregion where no protrusions are formed is present on the negativeelectrode current collector in a direction approximately parallel to thefirst width direction.

Hereinafter, the longitudinal direction of the first band-shaped regionis referred to as the third longitudinal direction. More specifically,on two ends in the longitudinal direction of the first band-shapedregion, the line connecting the midpoints in the respective widthdirections is defined as the third center line, and the direction of thethird center line is defined as the third longitudinal direction. Themidpoint in the width direction in each end can be determined for, forexample, a maximum rectangular band-shaped region virtually formedbetween the ends of adjacent protrusions. On such an occasion, theabove-described direction approximately parallel to the firstlongitudinal direction is defined such that the third longitudinaldirection and the first longitudinal direction are parallel to eachother and that the acute angle formed by the third longitudinaldirection and the first longitudinal direction is 30° or less.

The longitudinal direction of the second band-shaped region is referredto as the fourth longitudinal direction. More specifically, on two endsin the longitudinal direction of the second band-shaped region, the lineconnecting the midpoints in the respective width directions is definedas the fourth center line, and the direction of the fourth center lineis defined as the fourth longitudinal direction. The above-describeddirection approximately parallel to the first width direction is definedsuch that the fourth longitudinal direction and the first widthdirection are parallel to each other and that the acute angle formed bythe fourth longitudinal direction and the first width direction is 30°or less.

In each of the first surface and the second surface of the negativeelectrode current collector, as needed, for example, another regionwhere no protrusions are disposed may be provided in the innermostwinding portion and/or the outermost winding portion. That is, in thenegative electrode current collector, a region where no firstprotrusions and/or second protrusions are formed may be provided in aportion closest to the winding axis of the electrode group and/or aportion farthest from the winding axis of the electrode group. Anegative electrode lead for electrically connecting the negativeelectrode may be connected to the portion where no protrusions areformed on the first surface or the second surface of the negativeelectrode current collector by, for example, welding.

FIG. 1 is a plan view schematically illustrating a negative electrodeused in a lithium secondary battery according to an embodiment. In FIG.1, one surface of the negative electrode is shown. FIG. 2A is across-sectional view of the section taken along the line IIA-IIA in FIG.1 viewed from the direction of the arrow. FIG. 2B is a cross-sectionalview of the section taken along the line IIB-IIB in FIG. 1 viewed fromthe direction of the arrow.

The negative electrode 134 includes a negative electrode currentcollector 132 made of, for example, metal foil, outer-circumference-sideprotrusions 133A and inner-circumference-side protrusions 133Bprotruding from the surfaces of the negative electrode current collector132. The projection shape of each of the protrusions projected onto thesurface of the negative electrode current collector 132 in the thicknessdirection of the negative electrode current collector 132 is the same asthe shape of the planar view of the protrusions shown in FIG. 1, i.e., aline shape. When the surface of the negative electrode current collector132 shown in FIG. 1 is the first surface, the protrusions are the firstprotrusions; and when the surface is the second surface, the protrusionsare the second protrusions.

The surface of the negative electrode current collector 132 viewed fromthe normal direction has a rectangle shape in which the length in thedirection perpendicular to the winding axis when the electrode group isformed by winding is longer than the length in the direction parallel tothe winding axis. In FIG. 1, on the surface of the negative electrodecurrent collector 132, the direction perpendicular to the winding axisis indicated by the first longitudinal direction LD1, and the directionparallel to the winding axis is indicated by the first width directionWD1.

In FIG. 1, protrusions are provided on the surface of the negativeelectrode current collector 132 in such a manner that the secondlongitudinal direction LD2 of each protrusion is parallel to the firstlongitudinal direction LD1. The negative electrode current collector 132is divided into a region located in the inner winding portion IW of theelectrode group and a region located in the outer winding portion OW bythe center line CL dividing the length of the region facing the positiveelectrode active material of the negative electrode current collector132 into two parts in the first longitudinal direction LD1.

As shown in FIGS. 2A and 2B, the protrusions provided in the innerwinding portion IW have a width narrower than that of the protrusionsprovided in the outer winding portion OW. Accordingly, the area rate ofthe inner-circumference-side protrusions 133B to the inner windingportion IW is smaller than the area rate of the outer-circumference-sideprotrusions 133A to the outer winding portion OW. Consequently, even ifthe thickness of lithium metal deposited on the inner winding portion IWof the electrode group becomes large, the change in the volume caused bythe increase in the thickness can be absorbed. Accordingly, increase ofthe apparent volume of the negative electrode can be reduced.

On the surface of the outer winding portion OW, a first band-shapedregion 132 a where no outer-circumference-side protrusions 133A areformed is provided along the first longitudinal direction LD1. The thirdlongitudinal direction LD3 of the first band-shaped region 132 a isparallel to the first longitudinal direction LD1. In addition, in eachof the first surface and the second surface of the negative electrodecurrent collector 132, a second band-shaped region 132 b where noprotrusions are formed is provided at the center line CL and in thevicinity thereof. The fourth longitudinal direction LD4 of the secondband-shaped region 132 b is parallel to the first width direction WD1.On the surface of the inner winding portion IW, another firstband-shaped region 132 c where no inner-circumference-side protrusions133B are formed is provided along the third longitudinal direction LD3of the first band-shaped region 132 a.

The minimum clearance between two adjacent outer-circumference-sideprotrusions 133A is larger than the maximum width of the two adjacentouter-circumference-side protrusions 133A, and the minimum clearancebetween two adjacent inner-circumference-side protrusions 133B is largerthan the maximum width of the two adjacent inner-circumference-sideprotrusions 133B.

When a wound electrode group is formed by winding such a negativeelectrode 134 together with a positive electrode and a separator fromthe end on the inner winding portion IW side and is used in a lithiumsecondary battery, a space is formed between the negative electrodecurrent collector 132 and the separator between two adjacentprotrusions. The lithium metal deposited by charging is accommodated inthis space to reduce expansion of the negative electrode 134.

FIG. 3 is a plan view schematically illustrating another negativeelectrode used in a lithium secondary battery according to anembodiment. In FIG. 3, one surface of the negative electrode is shown.FIG. 4A is a cross-sectional view of the section taken along the lineIVA-IVA in FIG. 3 viewed from the direction of the arrow. FIG. 4B is across-sectional view of the section taken along the line IVB-IVB in FIG.3 viewed from the direction of the arrow.

As shown in FIGS. 4A and 4B, the width of the outer-circumference-sideprotrusions 133A provided in the outer winding portion OW and the widthof the inner-circumference-side protrusions 133B provided in the innerwinding portion IW (i.e., the length in the direction perpendicular tothe second longitudinal direction LD2) are the same. The number of theinner-circumference-side protrusions 133B is smaller than that of theouter-circumference-side protrusions 133A. Accordingly, the area rate ofthe protrusions in the inner winding portion IW is lower than that ofthe protrusions in the outer winding portion OW. The negative electrodeof FIG. 3 has the same configuration as that of the negative electrodeof FIG. 1, excepting the above.

For example, the whole or a part of the characteristics of protrusions,such as projection shapes, height, number, direction, width, andclearance between two adjacent protrusions, are not limited to thoseshown in FIGS. 1 and 3 and can be modified. These characteristics may bethe same or different in the inner winding portion IW and the outerwinding portion OW. When the protrusions are formed on both the firstsurface and the second surface of the negative electrode currentcollector 132, these characteristics may be the same or different on thefirst surface side and the second surface side of the negative electrodecurrent collector 132. In such a case, the protrusions disposed on thefirst surface side (i.e., the first protrusions) and the protrusionsdisposed on the second surface side (i.e., the second protrusions) maybe disposed in a zigzag manner when viewed from the normal direction ofthe first surface. That is, the protrusions may be disposed in such amanner that one second protrusion is disposed between two adjacent firstprotrusions.

The negative electrode 134 includes a negative electrode currentcollector 132 (e.g., a conductive sheet such as metal foil) andprotrusions formed on each surface of the conductive sheet. In the woundelectrode group, the outer surface and the inner surface of the negativeelectrode current collector 132 are, respectively, the first surface andthe second surface of the negative electrode current collector 132.

The conductive sheet is made of, for example, a conductive materialother than lithium metal and lithium alloys. The conductive material maybe a metal or a metal material such as an alloy or may be a carbonmaterial. The metal material may be a material that does not react withlithium. Examples of such a material include materials that do not reactwith lithium metal and/or lithium ions. More specifically, the materialmay be a material that does not form an alloy and an intermetalliccompound with lithium. As the carbon material, for example, graphitepreferentially exposing the basal plane can be used. Examples of themetal material include copper (Cu), nickel (Ni), iron (Fe), and alloyscontaining these metal elements. As the alloy, for example, a copperalloy or stainless steel may be used. From the viewpoint of easilyproviding a high strength, at least one selected from the groupconsisting of copper, copper alloys, and stainless steel may be used asthe conductive material. From the viewpoint of easily achieving a highcapacity and a high charge-discharge efficiency due to its highelectrical conductivity, the conductive material may be copper and/or acopper alloy. These conductive materials may be contained in theconductive sheet alone, or two or more thereof may be contained.

As the conductive sheet, for example, foil or film is used, and a sheetmade of a carbon material as described above may be used. The conductivesheet may be porous provided that the winding property is not impaired.From the viewpoint of easily securing a high conductivity, theconductive sheet may be metal foil or metal foil containing copper. Suchmetal foil may be copper foil or copper alloy foil. The content ofcopper in the metal foil may be 50 mass % or more or 80 mass % or more.In particular, the metal foil may be copper foil containingsubstantially copper only as the metal element.

Since protrusions are provided on the negative electrode 134, the firstsurface and the second surface of the negative electrode currentcollector 132 may be smooth. Consequently, lithium metal tends to beuniformly deposited on the first surface and the second surface of thenegative electrode current collector 132 during charging. The term“smooth” means that the maximum height roughness Rz of each of the firstsurface and the second surface of the negative electrode currentcollector 132 is 20 μm or less. The maximum height roughness Rz of eachof the first surface and the second surface of the negative electrodecurrent collector 132 may be 10 μm or less. The maximum height roughnessRz is measured in accordance with JIS B 0601:2013.

From the viewpoint of easily securing a high volumetric energy density,the negative electrode 134 may include only the negative electrodecurrent collector 132 and the protrusions when the lithium secondarybattery is in a fully discharged state. From the viewpoint of easilysecuring a high charge-discharge efficiency, in a fully dischargedstate, the negative electrode may include a negative electrode activematerial layer disposed on the surface of the negative electrode currentcollector, in addition to the negative electrode current collector 132and the protrusions.

In the present disclosure, the fully discharged state of a lithiumsecondary battery is a state obtained by discharging until a state ofcharge (SOC) of 0.05 C or less, where C denotes the rated capacity ofthe battery, and is, for example, a state obtained by discharging untilthe lower limit voltage at a constant current of 0.05 C. The lower limitvoltage is 2.5 V, for example.

Examples of the negative electrode active material contained in thenegative electrode active material layer include lithium metal, lithiumalloys, and materials that reversibly occlude and release lithium ions.The negative electrode active material may be that used in lithium-ionbatteries. Examples of the lithium alloy include a lithium aluminumalloy. Examples of the material that reversibly occlude and releaselithium ions include carbon materials and alloy materials. The carbonmaterial is, for example, at least one selected from the groupconsisting of graphite materials, soft carbon, hard carbon, andamorphous carbon. Examples of the alloy material include materialscontaining silicon and/or tin. The alloy material is, for example, atleast one selected from the group consisting of silicon simplesubstance, silicon alloys, silicon compounds, tin simple substance, tinalloys, and tin compounds. Examples of each of the silicon compound andthe tin compound are oxides and/or nitrides.

The negative electrode active material layer may be formed by depositinga negative electrode active material on the surface of the negativeelectrode current collector by a gas phase method such aselectrodeposition or vapor deposition. Alternatively, the negativeelectrode active material layer may be formed by applying a negativeelectrode mixture including a negative electrode active material, abinder, and, as needed, one or more components to the surface of thenegative electrode current collector. The component used as needed is,for example, at least one selected from the group consisting of aconductive agent, a thickener, and an additive. The negative electrodeactive material layer may have any thickness, and the thickness is, forexample, 1 μm or more and 150 μm or less per one surface of the negativeelectrode current collector in a fully discharged state of the lithiumsecondary battery.

The negative electrode active material layer and the protrusions may beformed in any order. The protrusions may be formed after formation ofthe negative electrode active material layer, or the negative electrodeactive material layer may be formed after formation of the protrusions.More specifically, protrusions being in direct contact with the firstsurface and/or the second surface of the negative electrode currentcollector 132 may protrude from each surface. Alternatively, protrusionsmay protrude from the first surface and/or the second surface of thenegative electrode current collector 132 with the negative electrodeactive material layer between each surface and the protrusions.

The negative electrode current collector or the conductive sheet mayhave any thickness, and the thickness is, for example, 5 μm or more and20 μm or less.

The protrusions may be made of any material. The material of theprotrusions may be different from the material of the negative electrodecurrent collector 132. Alternatively, the protrusions and the negativeelectrode current collector 132 may be integrally made of the samematerial. The protrusions may be each made of a conductive materialand/or an insulating material. The conductive material can beappropriately selected from those mentioned as the material for theconductive sheet. The negative electrode current collector 132 includingsuch protrusions can be prepared by forming the protrusions on thesurface of the conductive sheet by, for example, pressing.Alternatively, the negative electrode current collector 132 may beformed by applying coating or sticking tape of the conductive materialto the surface of the conductive sheet.

The protrusions may be each made of a resin material. The resin materialmay be insulative. Deposition of lithium metal on the tips of theprotrusions by charging can be reduced by making the protrusions from aninsulating material such as a resin material. The deposited lithiummetal is accommodated on the negative electrode 134, more preferably, inthe space formed in the vicinity of the surface of the negativeelectrode current collector 132, which is a conductive sheet such asmetal foil. Accordingly, the effect of reducing expansion of thenegative electrode can be enhanced.

The resin material is, for example, at least one selected from the groupconsisting of olefin resins, acrylic resins, polyamide resins, polyimideresins, and silicone resins. The resin material may be a cured productof a curable resin, such as an epoxy resin. The protrusions can beformed by, for example, attaching adhesive resin tape to the surface ofthe negative electrode current collector 132 or to the negativeelectrode active material layer disposed on the surface of the negativeelectrode current collector 132. Alternatively, the protrusions may beformed by applying a solution or dispersion containing a resin materialonto the surface of the negative electrode current collector 132 or thenegative electrode active material layer and drying it. The protrusionsalso can be formed by applying a curable resin into a desired shape onthe surface of the negative electrode current collector 132 and curingit.

The negative electrode 134 can further include a protective layer. Theprotective layer may be formed on the surface of the negative electrodecurrent collector 132. When the negative electrode 134 includes anegative electrode active material layer, the protective layer may beformed on the surface of the negative electrode active material layer.The protective layer has an effect of making the surface reaction of theelectrode more uniform. Accordingly, lithium metal tends to be moreuniformly deposited on the negative electrode. The protective layer canbe made of, for example, an organic material and/or an inorganicmaterial. As such a material, a material that does not inhibitlithium-ion conductivity is used. Examples of the organic materialinclude lithium-ion conductive polymers. Examples of the inorganicmaterial include ceramics and solid electrolytes.

Lithium Secondary Battery

A structure of the lithium secondary battery will now be morespecifically described. The lithium secondary battery includes a woundelectrode group and a nonaqueous electrolyte. The wound electrode groupis formed by winding a positive electrode, a negative electrode, and aseparator disposed between the electrodes.

FIG. 5 is a longitudinal cross-sectional view schematically illustratinga lithium secondary battery according to an embodiment of the presentdisclosure. FIG. 6 is an enlarged cross-sectional view schematicallyillustrating the region indicated by VI in FIG. 5. FIG. 7 is an enlargedcross-sectional view schematically illustrating the region indicated byVII in FIG. 5. FIG. 7 shows a section in a fully discharged state.

The lithium secondary battery 10 is a cylindrical battery including acylindrical battery case, a wound electrode group 14 accommodated in thebattery case, and a nonaqueous electrolyte (not shown). The battery caseis composed of a case body 15 which is a bottomed cylindrical metalcontainer and a sealing body 16 which seals the opening of the case body15. A gasket 27 is disposed between the case body 15 and the sealingbody 16 and secures sealability of the battery case. In the case body15, insulating plates 17, 18 are respectively disposed at both ends ofthe electrode group 14 in the winding axis direction.

The case body 15 has, for example, a stepped portion 21 formed bypartially pressing the side wall of the case body 15 from the outside.The stepped portion 21 may be annularly formed on the side wall of thecase body 15 along the circumferential direction of the case body 15. Insuch a case, the sealing body 16 is supported by the surface of thestepped portion on the opening side.

The sealing body 16 includes a filter 22, a lower valve 23, aninsulating member 24, an upper valve 25, and a cap 26. In the sealingbody 16, these members are stacked in this order. The sealing body 16 isset to the opening of the case body 15 such that the cap 26 is locatedoutside the case body 15 and the filter 22 is located inside the casebody 15. Each member constituting the sealing body 16 has, for example,a disk shape or a ring shape. The members excluding the insulatingmember 24 are electrically connected to each other.

The electrode group 14 includes a positive electrode 11, a negativeelectrode 12, and a separator 13. The positive electrode 11, thenegative electrode 12, and the separator 13 all have band-like shapes.The positive electrode 11 and the negative electrode 12 are spirallywound with the separator 13 between these electrodes such that the widthdirection of the band-shaped positive electrode 11 and negativeelectrode 12 is parallel to the winding axis. In a cross-sectionperpendicular to the winding axis of the electrode group 14, thepositive electrode 11 and the negative electrode 12 are alternatelylaminated in the radius direction of the electrode group 14 with theseparator 13 between these electrodes.

The positive electrode 11 is electrically connected to a cap 26, whichalso serves as a positive electrode terminal, via a positive electrodelead 19. One end of the positive electrode lead 19 is connected to thepositive electrode 11, for example, at the vicinity of the center in thelongitudinal direction. The positive electrode lead 19 extending fromthe positive electrode 11 passes through a through hole (not shown)formed in the insulating plate 17 and reaches the filter 22. The otherend of the positive electrode lead 19 is welded to the surface of thefilter 22 on the electrode group 14 side.

The negative electrode 12 is electrically connected to the case body 15,which also serves as a negative electrode terminal, via a negativeelectrode lead 20. One end of the negative electrode lead 20 isconnected to, for example, an end of the negative electrode 12 in thelongitudinal direction, and the other end is welded to the inner surfaceof the bottom of the case body 15.

FIG. 6 shows the positive electrode 11 facing the separator 13. FIG. 7shows the negative electrode 12 facing the separator 13. The positiveelectrode 11 includes a positive electrode current collector 30 andpositive electrode mixture layers 31 disposed on both the first surfaceand the second surface of the positive electrode current collector 30.The negative electrode 12 includes a negative electrode currentcollector 32, first protrusions 33 a disposed on the first surface S1 ofthe outer side of the negative electrode current collector 32, andsecond protrusions 33 b disposed on the second surface S2 of the innerside of the negative electrode current collector 32. The first surfaceS1 and the second surface S2 of the negative electrode current collector32 are, respectively, the first surface and the second surface of aconductive sheet, such as metal foil, constituting the negativeelectrode current collector 32. The first protrusions 33 a protrude fromthe first surface S1 toward the surface of the separator 13 facing thefirst surface S1. The second protrusions 33 b protrude from the secondsurface S2 toward the surface of the separator 13 facing the secondsurface S2.

The first protrusions 33 a and the second protrusions 33 b are formed onthe first surface S1 and the second surface S2, respectively. A space 35is formed between the first surface S1 and the separator 13 between twoadjacent first protrusions 33 a. In addition, a space 35 is formedbetween the second surface S2 and the separator 13 between two adjacentsecond protrusions 33 b. In the lithium secondary battery 10, lithiummetal is deposited in the space 35 by charging, and the depositedlithium metal is dissolved in the nonaqueous electrolyte by discharging.Since the space 35 can accommodate the deposited lithium metal, thechange in the apparent volume of the negative electrode 12 by depositionof lithium metal can be decreased. In addition, the change in volume bydeposition of lithium metal can be absorbed by controlling the area rateof the protrusions of the inner winding portion IW where the thicknessof deposited lithium metal increases to be smaller than the area rate ofthe protrusions of the outer winding portion OW in advance. Accordingly,the expansion of the negative electrode can be reduced. Furthermore, inthe electrode group 14, since a pressure is also applied to the lithiummetal accommodated in the space 35, peeling of the lithium metal isreduced. Accordingly, deterioration of the charge-discharge efficiencyof the lithium secondary battery 10 can also be reduced.

As the negative electrode 12 including protrusions and the negativeelectrode current collector 32, the negative electrode 134 including theabove-described protrusions and the negative electrode current collector132 can be used. Accordingly, regarding the negative electrode 12, theprotrusions, and the negative electrode current collector 32, theexplanation of the negative electrode 134, the protrusions, and thenegative electrode current collector 132 above can be referred to. Thestructure other than the negative electrode 12 of the lithium secondarybattery will now be described more specifically.

Positive Electrode 11

The positive electrode 11 includes, for example, a positive electrodecurrent collector 30 and a positive electrode mixture layer 31 formed onthe positive electrode current collector 30. The positive electrodemixture layer 31 may be formed on both the first surface and the secondsurface of the positive electrode current collector 30. The positiveelectrode mixture layer 31 may be formed one of the surfaces of thepositive electrode current collector 30. For example, in the regionconnected to the positive electrode lead 19 and/or the region not facingthe negative electrode 12, the positive electrode mixture layer 31 maybe formed on only one surface of the positive electrode currentcollector 30. For example, in the region of the innermost circumferenceof the winding and the vicinity thereof and/or the region of theoutermost circumference of the winding and the vicinity thereof, aregion not facing the negative electrode 12 may be present. Accordingly,in such a region, the positive electrode mixture layer 31 may be formedon only one surface of the positive electrode current collector 30, andthe positive electrode mixture layer 31 need not be formed on both thefirst surface and the second surface.

The positive electrode mixture layer 31 contains a positive electrodeactive material and can contain a conductive material and/or a binder asoptional components. The positive electrode mixture layer 31 may containan additive as needed. A conductive carbon material may be disposedbetween the positive electrode current collector 30 and the positiveelectrode mixture layer 31 as needed.

The positive electrode 11 is prepared by, for example, applying a slurrycontaining structural components of the positive electrode mixture layerand a dispersion medium to the surface of the positive electrode currentcollector 30, and drying and then rolling the resulting coating film. Aconductive carbon material may be applied onto the surface of thepositive electrode current collector 30 as needed. The dispersion mediumis, for example, water and/or an organic medium.

The positive electrode active material is, for example, a material thatoccludes and releases lithium ions. The positive electrode activematerial is, for example, at least one selected from the groupconsisting of lithium-containing transition metal oxides, transitionmetal fluorides, polyanions, fluorinated polyanions, and transitionmetal sulfides. The positive electrode active material may be alithium-containing transition metal oxide from the viewpoint of a highaverage discharge voltage and a cost advantage.

Examples of the transition metal element contained in thelithium-containing transition metal oxide include Sc, Ti, V, Cr, Mn, Fe,Co, Ni, Cu, Y, Zr, and W. The lithium-containing transition metal oxidemay contain a single transition metal element or may contain two or moretransition metal elements. The transition metal element may be at leastone selected from the group consisting of Co, Ni, and Mn. Thelithium-containing transition metal oxide can contain one or moretypical metal elements as needed. Examples of the typical metal elementinclude Mg, Al, Ca, Zn, Ga, Ge, Sn, Sb, Pb, and Bi. The typical metalelement may be, for example, Al.

The conductive material is, for example, a carbon material. Examples ofthe carbon material include carbon black, carbon nanotubes, andgraphite. Examples of the carbon black include acetylene black andKetjen black. The positive electrode mixture layer 31 may contain one ormore conductive materials. At least one material selected from thesecarbon materials may be used as a conductive carbon material lyingbetween the positive electrode current collector 30 and the positiveelectrode mixture layer 31.

Examples of the binder include fluororesins, polyacrylonitrile,polyimide resins, acrylic resins, polyolefin resins, and rubber-likepolymers. Examples of the fluororesin include polytetrafluoroethyleneand polyfluorinated vinylidene. The positive electrode mixture layer 31may contain a single binder or two or more binders.

Examples of the material of the positive electrode current collector 30include metal materials containing Al, Ti, Fe, or the like. The metalmaterial may be, for example, Al, an Al alloy, Ti, a Ti alloy, or a Fealloy. The Fe alloy may be stainless steel. Examples of the positiveelectrode current collector 30 include foil and film. The positiveelectrode current collector 30 may be porous. For example, metal meshmay be used as the positive electrode current collector 30.

Separator 13

As the separator 13, a porous sheet having ionic permeability andinsulation is used. Examples of the porous sheet include microporousfilm, woven fabric, and nonwoven fabric. The material of the separatoris not particularly limited and may be a polymer material. Examples ofthe polymer material include olefin resins, polyamide resins, andcellulose. Examples of the olefin resin include polyethylene,polypropylene, and copolymers of ethylene and propylene. The separator13 may contain an additive as needed. Examples of the additive includeinorganic fillers.

The separator 13 may include a plurality of layers having differentforms and/or compositions. Such separator 13 may be, for example, alayered product of a polyethylene microporous film and a polypropylenemicroporous film or a layered product of a nonwoven fabric containingcellulose fibers and a nonwoven fabric containing a thermoplastic resinfibers. Alternatively, the separator 13 may include a coating filmformed by applying a polyamide resin to a surface of, for example,microporous film, woven fabric, or nonwoven fabric. Such separator 13shows high durability even if a pressure is applied to the separator 13in a state being in contact with protrusions. In addition, from theviewpoint of securing heat resistance and/or strength, the separator 13may include a layer containing an inorganic filler on the side facingthe positive electrode 11 and/or the side facing the negative electrode12.

Nonaqueous Electrolyte

The nonaqueous electrolyte to be used has lithium-ion conductivity. Sucha nonaqueous electrolyte includes a nonaqueous solvent and lithium ionsand anions dissolved in the nonaqueous solvent. The nonaqueouselectrolyte may be in a liquid form or may be in a gel form. Thenonaqueous electrolyte may be a solid electrolyte.

The nonaqueous electrolyte in a liquid form is prepared by dissolving alithium salt in a nonaqueous solvent. Lithium ions and anions aregenerated by that the lithium salt is dissolved in the nonaqueoussolvent. The nonaqueous electrolyte may contain an undissociated lithiumsalt. As the lithium salt, a salt of a lithium ion and an anion is used.

The nonaqueous electrolyte in a gel form include a liquid nonaqueouselectrolyte and a matrix polymer. The matrix polymer to be used is, forexample, a polymer material that absorbs a nonaqueous solvent andthereby gels. Such a polymer material is, for example, at least oneselected from the group consisting of fluororesins, acrylic resins, andpolyether resins.

As the lithium salt and the anion, known lithium salts and anions thatare used for nonaqueous electrolytes of lithium secondary batteries canbe used. Examples of the anion include BF₄ ⁻, ClO₄ ⁻, PF₆ ⁻, CF₃SO₃ ⁻,CF₃CO₂ ⁻, anions of imides, and anions of oxalates. Examples of theanion of an imide include N(SO₂C_(m)F_(2m+1))(SO₂C_(n)F_(2n+1))⁻ (wherem and n are each independently an integer of 0 or more). In the formula,m and n may be each 0 to 3 or may be each 0, 1, or 2. The anion of animide may be N(SO₂CF₃)₂ ⁻, N(SO₂C₂F₅)₂ ⁻, or N(SO₂F)₂ ⁻. The anion of anoxalate may contain boron and/or phosphorus. The anion of an oxalate maybe an anion of an oxalate complex. Examples of the anion of an oxalateinclude bis(oxalate)borate anions, BF₂(C₂O₄)⁻, PF₄(C₂O₄)⁻, andPF₂(C₂O₄)₂ ⁻. The nonaqueous electrolyte may include a single type ortwo or more types of these anions.

From the viewpoint of reducing deposition of lithium metal in a dendriteform, the nonaqueous electrolyte may include at least one type selectedfrom the group consisting of PF₆ ⁻, anions of imides, and anions ofoxalates. In particular, by using a nonaqueous electrolyte containinganions of an oxalate, fine particles of lithium metal tend to be evenlydeposited due to interaction between the anions of the oxalate andlithium. Accordingly, it is possible to reduce uneven expansion of thenegative electrode due to local deposition of lithium metal. Acombination of anions of an oxalate complex and other anions may beused. Such other anions may be PF₆ ⁻ and/or anions of imides.

Examples of the nonaqueous solvent include esters, ethers, nitriles,amides, and halogen-substituted products thereof. The nonaqueouselectrolyte may include one of these nonaqueous solvents or two or morenonaqueous solvents. Examples of the halogen-substituted product includefluorides.

Examples of the ester include carbonates and carboxylates. Examples ofcyclic carbonate include ethylene carbonate, propylene carbonate, andfluoroethylene carbonate. Examples of chain carbonate include dimethylcarbonate, ethyl methyl carbonate, and diethyl carbonate. Examples ofcyclic carboxylate include γ-butyrolactone and γ-valerolactone. Examplesof chain carboxylate include ethyl acetate, methyl propionate, andmethyl fluoropropionate.

Examples of the ether include cyclic ethers and chain ethers. Examplesof the cyclic ether include 1,3-dioxolane, 4-methyl-1,3-dioxolane,tetrahydrofuran, and 2-methyltetrahydrofuran. Examples of the chainether include 1,2-dimethoxyethane, diethyl ether, ethyl vinyl ether,methyl phenyl ether, benzyl ethyl ether, diphenyl ether, dibenzyl ether,1,2-diethoxyethane, and diethylene glycol dimethyl ether.

Examples of the nitrile include acetonitrile, propionitrile, andbenzonitrile. Example of the amide include dimethylformamide anddimethylacetamide.

The concentration of the lithium salt in the nonaqueous electrolyte is,for example, 0.5 mol/L or more and 3.5 mol/L or less. Here, theconcentration of a lithium salt is the sum of the concentration ofdissociated lithium salt and the concentration of undissociated lithiumsalt. The concentration of anions in the nonaqueous electrolyte may be0.5 mol/L or more and 3.5 mol/L or less.

The nonaqueous electrolyte may contain an additive. The additive mayform a coating film on the negative electrode. Generation of dendritestends to be reduced by formation of a coating film of the additive onthe negative electrode. Examples of the additive include vinylenecarbonate, fluoroethylene carbonate, and vinyl ethylene carbonate. Theseadditives may be used alone or in combination of two or more thereof.

Others

In the examples shown in the drawings, cylindrical lithium secondarybatteries have been described, but the battery is not limited thereto,and the present embodiment can also be applied to lithium secondarybatteries including a wound electrode group of which the shape of theend face in the winding axis direction is elliptic or oval. Regardingthe structures other than the electrode group and the nonaqueouselectrolyte of the lithium secondary battery, any known structures canbe used without specific limitations.

EXAMPLES

The lithium secondary battery according to the present disclosure willnow be specifically described based on Examples and ComparativeExamples. The present disclosure is not limited to the followingExamples.

Example 1

(1) Production of Positive Electrode

A positive electrode active material, acetylene black as a conductivematerial, and polyfluorinated vinylidene as a binder were mixed at amass ratio of 95:2.5:2.5. An appropriate amount ofN-methyl-2-pyrrolidone as a dispersion medium was added to the mixture,followed by stirring to prepare a positive electrode mixture slurry. Thepositive electrode active material used was a lithium-containingtransition metal oxide containing Ni, Co, and Al.

The positive electrode mixture slurry was applied to both surfaces ofaluminum foil as a positive electrode current collector and was thendried. The dried product was compressed in the thickness direction witha roller. The resulting layered product was cut into a predeterminedelectrode size to produce a positive electrode provided with a positiveelectrode mixture layer on both surfaces of the positive electrodecurrent collector. An exposed portion of the positive electrode currentcollector not provided with the positive electrode mixture layer wasformed in a partial region of the positive electrode. One end of analuminum positive electrode lead was attached to the exposed portion ofthe positive electrode current collector by welding.

(2) Production of Negative Electrode

A negative electrode 134 including outer-circumference-side protrusions133A and inner-circumference-side protrusions 133B on a negativeelectrode current collector 132 was produced by the following procedure.An electrolytic copper foil having a thickness of 10 μm was used as anegative electrode current collector 132, and a polyethylene adhesivetape was attached to both the first surface and the second surface ofthe negative electrode current collector 132 to form a plurality ofline-shaped protrusions 133. More specifically, according to the caseshown in FIGS. 1, 2A, and 2B, on each of the first surface and thesecond surface of the negative electrode current collector 132, anadhesive tape having a thickness of 50 μm and a width of 2 mm wasattached to the region becoming the outer winding portion OW of theelectrode group, and an adhesive tape having a thickness of 50 μm and awidth of 1 mm was attached to the region becoming the inner windingportion IW. On this occasion, the adhesive tapes were attached such thatthe longitudinal direction of the adhesive tapes was parallel to thefirst longitudinal direction LD1 of the negative electrode currentcollector 132.

On each of the first surface and the second surface of the negativeelectrode current collector 132, the minimum value of thecenter-to-center distance of two adjacent outer-circumference-sideprotrusions 133A disposed on the outer winding portion OW was about 5mm. The first area rate of the outer-circumference-side protrusions 133Awas 40% (=(adhesive tape width: 2 mm/center-to-center distance: 5mm)×100).

On each of the first surface and the second surface of the negativeelectrode current collector 132, the minimum value of thecenter-to-center distance of two adjacent inner-circumference-sideprotrusions 133B disposed on the inner winding portion IW was about 5mm. The second area rate of the inner-circumference-side protrusions133B was 20% (=(adhesive tape width: 1 mm/center-to-center distance: 5mm)=100). The first average height of the outer-circumference-sideprotrusions 133A and the second average height of theinner-circumference-side protrusions 133B were both 50 μm.

The resulting product was cut into a predetermined electrode size toform a negative electrode 134 provided with a plurality of line-shapedprotrusions on each of the first surface and the second surface of thenegative electrode current collector 132. One end of a nickel negativeelectrode lead was attached to the negative electrode 134 by welding.

(3) Preparation of Nonaqueous Electrolyte

Ethylene carbonate and dimethyl carbonate were mixed at a volume ratioof 3:7. LiPF₆ and LiBF₂(C₂O₄)₂ were dissolved in the resulting solventmixture at concentrations of 1 mol/L and 0.1 mol/L, respectively, toprepare a liquid nonaqueous electrolyte.

(4) Production of Battery

In an inert gas atmosphere, the positive electrode prepared as describedin (1) and the negative electrode 134 prepared as described in (2) werestacked with a polyethylene microporous film as a separatortherebetween. More specifically, a positive electrode, a separator, anegative electrode 134, and a separator were stacked in this order. Theresulting layered product was spirally wound to produce an electrodegroup. On this occasion, the layered product was wound so that theregion of the negative electrode 134 where the protrusions having awidth of 1 mm were formed was the inner winding portion IW of theelectrode group. In the resulting electrode group, approximately 100% ofthe upper surfaces of the protrusions were in contact with theseparator. The electrode group was accommodated in a bag-like exteriorbody formed from a laminate sheet including an Al layer, the nonaqueouselectrolyte was poured in the exterior body accommodating the electrodegroup, and the exterior body was then sealed. A lithium secondarybattery T1 was thus produced.

Example 2

According to the case shown in FIGS. 3, 4A, and 4B, on each of the firstsurface and the second surface of the negative electrode currentcollector 132, an adhesive tape having a thickness of 50 μm and a widthof 1 mm was attached to the regions becoming the outer winding portionOW and the inner winding portion IW of the electrode group. Here, theminimum value of the center-to-center distance of two adjacentouter-circumference-side protrusions 133A disposed on the outer windingportion OW was set to be about 5 mm. The minimum value of thecenter-to-center distance of two adjacent inner-circumference-sideprotrusions 133B disposed on the inner winding portion IW was set to beabout 8.5 mm. A lithium secondary battery T2 was produced as in Example1, excepting the above.

The first area rate of the outer-circumference-side protrusions 133A was20% (=(adhesive tape width: 1 mm/center-to-center distance: 5 mm)×100).The second area rate of the inner-circumference-side protrusions 133Bwas 11.8% (=(adhesive tape width: 1 mm/center-to-center distance: 8.5mm)×100).

Example 3

A negative electrode including protrusions according to the case shownin FIGS. 3, 4A, and 4B and a lithium secondary battery T3 were producedas in Example 2 except that the thickness of the adhesive tape used wasof 35 mm.

Comparative Example 1

A lithium secondary battery R1 was produced as in Example 1 except thatthe negative electrode was produced by forming continuous protrusionsextending from one end to the other end of metal foil in thelongitudinal direction by a polyethylene adhesive tape having athickness of 50 μm and a width of 1 mm on both surfaces of the metalfoil. The minimum value of the center-to-center distance of theprotrusions was about 5 mm.

Comparative Example 2

A negative electrode formed as in Example 1 was used. However, theelectrode group was produced by winding the layered product in such amanner that the outer winding portion OW of the electrode group was theregion of the negative electrode provided with protrusions having awidth of 1 mm and that the inner winding portion IW of the electrodegroup was the region provided with protrusions having a width of 2 mm. Alithium secondary battery R2 was produced as in Example 1 excepting theabove.

Comparative Example 3

A negative electrode formed as in Example 2 was used. However, theelectrode group was produced by winding the layered product in such amanner that the outer winding portion OW of the electrode group was theregion of the negative electrode on which protrusions were disposed suchthat the minimum value of the center-to-center distance was about 8.5 mmand that the inner winding portion IW of the electrode group was theregion of the negative electrode on which protrusions were disposed suchthat the minimum value of the center-to-center distance was about 5 mm.A lithium secondary battery R3 was produced as in Example 2 exceptingthe above.

Evaluation

The lithium secondary batteries prepared in Examples and ComparativeExamples were subjected to a charge discharge test by the followingprocedure to evaluate expansion of the negative electrodes. The lithiumsecondary batteries were charged in a thermostatic chamber of 25° C.under the conditions described below, the charging was then paused for20 minutes, and the batteries were discharged under the conditionsdescribed below.

Charging

Constant-current charging was performed with a current of 10 mA per unitarea (unit: square centimeter) of the electrode until the batteryvoltage reached 4.3 V, and constant-voltage charging was then performedwith a voltage of 4.3 V until the current value per unit area (unit:square centimeter) of the electrode reached 1 mA.

Discharging

Constant-current discharging was performed with a current of 10 mA perunit area (unit: square centimeter) of the electrode until the batteryvoltage reached 2.5 V.

The above-described charging and discharging operation was defined asone cycle, and after the charging of the second cycle, the batterieswere each disassembled to take out the negative electrode. The negativeelectrode was cleaned with dimethyl carbonate and was dried, and thethickness of the negative electrode was then measured. The thickness ofthe negative electrode was determined by measuring the thicknesses atarbitrary five points in the negative electrode with a peacock digitalthickness gauge G2-205M and averaging the thicknesses. The thickness ofthe current collector of the negative electrode before the charging anddischarging operation was defined as 100%, and the rate (%) of thethickness of the negative electrode at the second cycle to the abovethickness of the current collector was defined as the negative electrodeexpansion coefficient.

The results of Examples and Comparative Examples are shown in Table 1.Table 1 also shows the first area rate of protrusions formed on thenegative electrode to the outer winding portion OW of the electrodegroup and the second area rate of protrusions to the inner windingportion IW.

TABLE 1 First area Second area Negative electrode rate (%) rate (%)expansion coefficient (%) T1 40 20 112 T2 20 11.8 115 T3 20 11.8 110 R120 20 138 R2 20 40 133 R3 11.8 20 130

As shown in Table 1, the negative electrode expansion coefficients inbatteries T1 to T3 of Examples were lower than those in batteries R1 toR3 of Comparative Examples. The difference between Examples andComparative Examples is only the area rate correlation between theprotrusions in each winding portion. In these batteries, a slightdifference in the area rate of protrusions causes a significantdifference in the negative electrode expansion coefficient betweenComparative Examples and Examples. It is conceived that in Examples,since the area rate of the protrusions provided in the inner windingportion IW of the electrode group is small, even if the thickness oflithium metal is increased, this increase can be absorbed, and theexpansion of the negative electrode is thereby reduced.

In the lithium secondary battery according to the present disclosure,since the expansion of the negative electrode can be reduced, a highdischarge capacity tends to be achieved. Accordingly, the lithiumsecondary battery according to the present disclosure is useful forvarious applications, for example, electronic devices, such as mobilephones, smart phones, and tablet terminals; electric vehicles includinghybrids and plug-in hybrids; and household storage batteries combinedwith solar batteries.

What is claimed is:
 1. A lithium secondary battery comprising: anonaqueous electrolyte having lithium-ion conductivity; and an electrodegroup including: a positive electrode containing a positive electrodeactive material containing lithium; a negative electrode including anegative electrode current collector and protrusions disposed on thenegative electrode current collector; and a separator disposed betweenthe positive electrode and the negative electrode, wherein: the positiveelectrode, the negative electrode, and the separator of the electrodegroup are wound, lithium metal is deposited on the negative electrodeduring charging, and the lithium metal is dissolved in the nonaqueouselectrolyte during discharging; the negative electrode current collectorhas a first surface and a second surface opposite to the first surface,the first surface includes a first region and a second region that iscloser to an innermost circumference of the winding of the electrodegroup than the first region, the second surface includes a third regionand a fourth region that is closer to the innermost circumference of thewinding of the electrode group than the third region, the protrusionsinclude outer-circumference-side protrusions disposed on the firstregion and inner-circumference-side protrusions disposed on the secondregion, and in the first surface, a first area rate is larger than asecond area rate, where the first area rate refers to(A_(OP)/A_(O))×100%, A_(OP) refers to a sum of the projection areas ofthe outer-circumference-side protrusions onto the first region, A_(O)refers to an area of the first region, the second area rate refers to(A_(IP)/A_(I))×100%, A_(IP) refers to a sum of the projection areas ofthe inner-circumference-side protrusions onto the second region, andA_(I) refers to an area of the second region.
 2. The lithium secondarybattery according to claim 1, wherein: in the first surface, the firstregion is far from the innermost circumference than a center line in alongitudinal direction of the first surface; and in the first surface,the second region is closer to the innermost circumference than thecenter line of the first surface.
 3. The lithium secondary batteryaccording to claim 1, wherein a difference between the first area rateand the second area rate is 3 percentage points or more and 50percentage points or less.
 4. The lithium secondary battery according toclaim 1, wherein the first area rate is 0.2% or more and 70% or less. 5.The lithium secondary batter according to claim 1, wherein a differencebetween a first average height of the outer-circumference-sideprotrusions and a second average height of the inner-circumference-sideprotrusions is less than 3% of the second average height.
 6. The lithiumsecondary battery according to claim 1, wherein a first average heightof the outer-circumference-side protrusions is 15 μm or more and 120 μmor less; and a second average height of the inner-circumference-sideprotrusions is 15 μm or more and 120 μm or less.
 7. The lithiumsecondary battery according to claim 1, wherein: in the first surface,projection shapes of the outer-circumference-side protrusions onto thefirst region are line shapes, in the first surface, projection shapes ofthe inner-circumference-side protrusions onto the second region are lineshapes, in the first surface, a minimum clearance between two adjacentouter-circumference-side protrusions out of the outer-circumference-sideprotrusions is larger than a maximum width of the two adjacentouter-circumference-side protrusions, and in the first surface, aminimum clearance between two adjacent inner-circumference-sideprotrusions out of the inner-circumference-side protrusions is largerthan a maximum width of the two adjacent inner-circumference-sideprotrusions.
 8. The lithium secondary battery according to claim 1,wherein the negative electrode current collector includes copper foil orcopper alloy foil.
 9. The lithium secondary battery according to claim1, wherein the protrusions are in contact with the separator; and thelithium metal is deposited in a space between the negative electrodecurrent collector and the separator during charging.
 10. The lithiumsecondary battery according to claim 1, wherein the protrusions are madeof a material different from a material of the negative electrodecurrent collector.
 11. The lithium secondary battery according to claim1, the protrusions are made of a resin material.
 12. The lithiumsecondary battery according to claim 1, wherein the negative electrodecurrent collector and the protrusions are integrally made of a samematerial.
 13. The lithium secondary battery according to claim 1,wherein: the protrusions include outer-circumference-side protrusionsdisposed on the third region and inner-circumference-side protrusionsdisposed on the fourth region, the outer-circumference-side protrusionsinclude first protrusions disposed on the first surface and secondprotrusions disposed on the second surface, and theinner-circumference-side protrusions include first protrusions disposedon the first surface and second protrusions disposed on the secondsurface.
 14. The lithium secondary battery according to claim 13,wherein a difference between an average height of the first protrusionsof the outer-circumference-side protrusions and/or theinner-circumference-side protrusions and an average height of the secondprotrusions of the outer-circumference-side protrusions and/or theinner-circumference-side protrusions is less than 3% of the averageheight of the second protrusions.
 15. The lithium secondary batteryaccording to claim 1, wherein the nonaqueous electrolyte includeslithium ions and anions; and the anions include at least one type ofanion selected from the group consisting of PF₆ ⁻, anions of imides, andanions of oxalates.
 16. A lithium secondary battery comprising: anonaqueous electrolyte having lithium-ion conductivity; and an electrodegroup including: a positive electrode containing a positive electrodeactive material containing lithium; a negative electrode including anegative electrode current collector and protrusions disposed on thenegative electrode current collector; and a separator disposed betweenthe positive electrode and the negative electrode, wherein: the positiveelectrode, the negative electrode, and the separator of the electrodegroup are wound, lithium metal is deposited on the negative electrodeduring charging, and the lithium metal is dissolved in the nonaqueouselectrolyte during discharging, the negative electrode current collectorhas a first surface facing outward of the winding of the electrode groupand a second surface facing inward of the winding of the electrodegroup, the first surface includes a first region and a second regionthat is closer to an innermost circumference of the winding of theelectrode group than the first region, the second surface includes athird region and a fourth region that is closer to the innermostcircumference of the winding of the electrode group than the thirdregion, the protrusions include first outer-circumference-sideprotrusions disposed on the first region and firstinner-circumference-side protrusions disposed on the second region, theprotrusions include second outer-circumference-side protrusions disposedon the third region and second inner-circumference-side protrusionsdisposed on the fourth region, and a first area rate is larger than asecond area rate, where the first area rate refers to an arithmetic meanof a third area rate and a fourth area rate, the third area rate refersto (A_(O1P)/A_(O1))×100%, A_(O1P) refers to a sum of the projectionareas of the first outer-circumference-side protrusions in the firstsurface onto the first region, A_(O1) refers to an area of the firstregion in the first surface, the fourth area rate refers to(A_(O2P)/A_(O2))×100%, A_(O2P) refers to a sum of the projection areasof the second outer-circumference-side protrusions in the second surfaceonto the third region, A_(O2) refers to an area of the third region inthe second surface, the second area rate refers to an arithmetic mean ofa fifth area rate and a sixth area rate, the fifth area rate refers to(A_(I1P)/A_(I1))×100%, A_(I1P) refers to a sum of the projection areasof the first inner-circumference-side protrusions in the first surfaceonto the second region, A_(I1) refers to an area of the second region inthe first surface, the sixth area rate refers (A_(I2P)/A_(I2))×100%,A_(I2P) refers to a sum of the projection areas of the secondinner-circumference-side protrusions in the second surface onto thefourth region, and A_(I2) refers to an area of the fourth region in thesecond surface.
 17. The lithium secondary battery according to claim 1,wherein the first surface faces outward of the winding of the electrodegroup and the second surface faces inward of the winding of theelectrode group.
 18. The lithium secondary battery according to claim 1,wherein the second surface faces outward of the winding of the electrodegroup and the first surface faces inward of the winding of the electrodegroup.